In nuclear emergencies such as nuclear accidents, nuclear terrors and nuclear wars, ionizing radiation can cause serious acute radiation injury (ARI) in humans. Treatment and prevention of the radiation-induced injuries concerns peaceful uses of atomic energy and national nuclear safety, and thus studies in this field has become a research field which receives high attention and significant investment from governments and scholars around the world.
Deinococcus radiodurans (DR) is a prokaryotic bacterium having the strongest radioresistance found on the earth to date, and the extremely strong radioresistance of this bacterium is associated with its own perfect and efficient DNA repair system, in which a plurality of DNA repair genes and proteins thereof play a crucial role in its specific radioresistance. In 1999, White et al. have firstly disclosed the gene sequence of DR, wherein an inducer of pleiotropic proteins promoting DNA repair (pprI) is one radioresistant gene having important regulatory effect in the Deinococcus radiodurans. PprI gene contains 987 bp, and encodes 328 amino acids (AAs), and from which the product PprI protein is encoded by DR_0167, with a molecular weight of 37 KD. Recent studies have shown that, pprI gene is the master switch gene for controlling the DNA repair and protection pathways of Deinococcus radiodurans; and after radiation of Deinococcus radiodurans, the PprI protein could up-regulate the expression of over 210 genes via multiple signaling pathways, in which 21 genes associated with DNA repair and replication are included [H Lu, H Chen, G Xu, et al. DNA Repair, 2012, 11(2):139-145]. In the past more than 50 years from the discovery of Deinococcus radiodurans, intensive studies on the function of the genes and proteins of Deinococcus radiodurans as well as the mechanisms thereof have been carried out by scholars around the world. However, to date, these studies only focus on prokaryotic cells, that is, Deinococcus radiodurans itself or Escherichia Coli. 
In Chinese patent application No. 200910003512.2, a eukaryotic expression recombinant plasmid pCMV-HA-pprI from Deinococcus radiodurans pprI gene was firstly constructed, which was transformed into a human embryo kidney HEK-293T cell and a radiated mammal to successfully express PprI protein, which has very significant preventive and therapeutic effect on fatal acute radiation injury in animals, suggesting that the PprI protein has potential to become a new biological agent for prevention and treatment of acute radiation injury. However, presently, from the eukaryotic expression recombinant plasmid pCMV-HA-pprI in this patent, it is still difficult to obtain efficiently and massively expressed and purified PprI protein via human cell engineering.
A prokaryotic expression system Escherichia Coli can be used for efficient expression and purification of PprI protein [ZHANG Yongqin, ZHOU Hui, CHEN Jie, YANG Zhanshan, JOURNAL OF RADIATION RESEARCH AND RADIATION PROCESSING, 2011, 29(2): 117-122]; however, such a method have many defects in that: 1. the expressed protein is not subjected to post-translational modification such as glycosylation, resulting in a decreased activity; 2. the expressed protein is present in the form of inclusion body, which also results in a decreased activity of the protein upon extraction through denaturation; 3. there may be endotoxins and toxic proteins from Escherichia Coli itself blended in the protein product of interest, resulting in unexpected toxicity.
Yeast is one of eukaryotic expression systems commonly used in gene engineering, among which Pichia pastoris is one of yeast engineering strains using methanol as the sole carbon source. Such yeast has the following advantages: 1. such yeast is simple in genetic manipulation, has a highly stable genome, and can express an exogenous gene at a high level; 2. such yeast can proceed post-translational modifications of a protein, such as glycosylation, disulfide bond formation, signal peptide cleavage, etc.; 3. the expression vector does not contain a yeast replication origin, the exogenous gene is homologously recombined into the chromosome of a yeast cell and exists stably, and the integrated exogenous gene can passage stably along with the growth of yeast.
If the Deinococcus radiodurans PprI protein can be expressed and purified by the eukaryotic expression system Pichia, the defects of expression and purification of PprI protein with a human cell or a prokaryotic expression system Escherichia Coli in the prior art will be improved or solved. However, Deinococcus radiodurans belongs to prokaryotic organism, which has huge differences in phylogenetic evolution from the eukaryotic organism Pichia, for example, significant differences in gene and protein composition and function, amino acid codon preference of a protein and other aspects. Therefore, if a Deinococcus radiodurans PprI gene is directly constructed into a Pichia expression system via gene engineering, as demonstrated by pre-experimental studies, it is impossible and also is adverse to efficiently express the Deinococcus radiodurans PprI protein, which problem cannot be successfully solved easily in accordance with the general genetic modification techniques in the art.
Therefore, the present invention provides a technology for successful expression and purification of PprI protein of Deinococcus radiodurans R1 via an eukaryotic expression system Pichia, which will establish a solid basis for further studies on functions, mechanisms and applications of this protein, make up for the blank that there has yet been no radioresistant prokaryotic protein drug in the field of treatment and prevention of radiation injury internationally, and rises to the internationally advanced ranks in studies on original protein drugs as radioprotective agents.